TEM of myelin sheath segment from transverse section of a frogs sciatic nerve. The image is demonstrating a concentric array of dense intermediate layers. Myelin is a fatty white substance that surrounds the axon of some nerve cells, forming an electrically insulating layer. It is essential for the proper functioning of the nervous system. It is an outgrowth of a type of glial cell. Magnification: 350,000x. The insert in the lower left corner is low-angle x-ray diffraction pattern of a rat sciatic nerve recorded with finean camera. This pattern features a fundamental period of 178A, with characteristic alteration of the intensities of the even and odd orders.